Many lighting devices use Light Emitting Diodes (LEDs) as a light emitter and often such a LED is provided with a luminescent material to obtain a specific light emission. A combination of a LED with a specific luminescent material to emit a specific color of light is termed a LED assembly in this background of the art section. In several lighting devices, LED assemblies of different types (that emit different colors of light) are used in a single device to obtain a specific light emission having, for example, a predefined color, a predefined color temperature, a predefined color rending index, a specific power output, and/or a specific wavelength distribution. Sometimes different LED assemblies are used that are controlled individually or controlled in groups to obtain a controllable light source of which, for example, the emitted color of light or the emitted color temperature can be controlled.
US 2013/0214302A1 discloses a substrate with a plurality of light emitting diode chips, comprising at least a first, a second and a third light emitting diode chip. The substrate comprises a plurality of different shaped concaves for proper placement of the light emitting diode chips.
Published U.S. Pat. No. 8,735,914, which is incorporated by reference, discloses a light emitting device in which a plurality of different types of LED assemblies are provided on a substrate. Groups of LEDs are formed, and each group comprises the same type of LED assembly. Within the groups, LEDs are placed close to each other and the groups are placed adjacent to each other. Electrical connections and electrodes are provided such that each group can be controlled individually. The substrate with LED assemblies is relatively small and can be arranged in a retrofit light bulb.
A problem of the latter cited patent is that the different light emissions of the different groups do not mix very well. Therefore, if no additional light mixing means is provided, the emitted light beam has different colors of light in different parts of the light beam. Therefore, the light emitting device of the cited patent can only be used in lamp designs where diffusers or light mixing chambers are available. However, there is a need to use different types of LED assemblies also in a spot lamp, for example in combination with a reflector. In particular, in spots, it is very important that different colors of light are well mixed into the emitted light beam to prevent, for example, that portions of the illuminated area appear to have different colors.